1. Field of the Invention
The present invention relates to a semiconductor light-emitting device and a method of producing the same, and more particularly to a semiconductor light-emitting device having a pad electrode that is opaque to light emitted from a semiconductor light-emitting layer, and a method of producing the same.
2. Description of the Background Art
An LED (Light Emitting Diode) chip which is a semiconductor light-emitting device produced using an AlGaInN-based nitride semiconductor can emit light of short wavelengths, such as blue, with high efficiency. Combining this LED chip and a phosphor provides a light-emitting apparatus that emits white light and the like. Such light-emitting apparatuses may exceed fluorescent lamps in luminous efficiency, and are considered to become the mainstream of future illumination. Accordingly, nitride semiconductor light-emitting devices are expected to be improved further in luminous efficiency and reliability and to be developed in energy saving.
In a nitride semiconductor light-emitting device with a nitride semiconductor layer formed on an insulation substrate, an electrode cannot be disposed at the rear surface side of the substrate. Therefore, a p-side electrode and an n-side electrode are disposed at the upper surface side of the substrate with the nitride semiconductor layer formed thereon.
The n-side electrode needs to be electrically connected to an n-type semiconductor layer. Therefore, to form the n-type semiconductor layer, a p-type nitride semiconductor layer and a semiconductor light-emitting layer need to be removed to expose an n-type nitride semiconductor layer. An unexposed section other than an exposed section having been exposed by etching represents a mesa (trapezoidal) form.
At the boundary between the unexposed section and the exposed section, an area where the ends of the n-type semiconductor layer, the semiconductor light-emitting layer and the p-type semiconductor layer are exposed will be referred to as an exposed end. Since the pn junction is exposed at the exposed end, a leakage current occurs when the pn junction in the exposed section is contaminated.
Japanese Patent Laying-Open No. 9-205224 (Patent Literature 1) is a conventional literature that discloses a nitride semiconductor light-emitting device having an insulation film in order to protect the exposed end. In the nitride semiconductor light-emitting device disclosed in Patent Literature 1, the insulation film is formed on a surface of the exposed end.
Since an opaque p-side pad electrode is disposed on the p-type nitride semiconductor layer, light emitted from the semiconductor light-emitting layer located under the p-side pad electrode is blocked by the p-side pad electrode from outgoing from the upper surface of the nitride semiconductor light-emitting device. Accordingly, the light extraction efficiency of the semiconductor light-emitting device deteriorates.
Japanese Patent Laying-Open No. 8-250769 (Patent Literature 2) is a conventional literature that discloses a semiconductor light-emitting device with a high resistance layer formed therein in order to reduce a value of current flowing through part of a p-type semiconductor layer that is located under a p-side pad electrode. In the semiconductor light-emitting device disclosed in Patent Literature 2, the high resistance layer is disposed between a transparent electrode and the p-type semiconductor layer.
Japanese Patent Laying-Open No. 2008-210903 (Patent Literature 3) is a conventional literature that discloses a semiconductor light-emitting device with a p-side pad electrode disposed on an insulation film with a light-transmissive conductive film interposed therebetween. In the semiconductor light-emitting device disclosed in Patent Literature 3, a light-transmissive insulation film is disposed between the light-transmissive conductive film having an external connection part and a p-type semiconductor layer.
Japanese Patent Laying-Open No. 2008-135554 (Patent Literature 4) is a conventional literature that discloses a semiconductor light-emitting device with a transparent conductive film formed also under a light-transmissive insulation layer. In the semiconductor light-emitting device disclosed in Patent Literature 4, the layer thickness of a portion of the transparent conductive film formed under the light-transmissive insulation layer is smaller than that of the remaining portion of the transparent conductive film. This structure uniformly distributes current while increasing light emission around a pad electrode, thereby improving light extraction efficiency.
In a structure having an insulation film under a p-side electrode as in the semiconductor light-emitting devices disclosed in Patent Literatures 2 and 3, an ohmic contact between a transparent conductive film and a p-type semiconductor layer is less likely to be obtained. To obtain the ohmic contact between the transparent conductive film and the p-type semiconductor layer, an oxide existing at the interface between the transparent conductive film and the p-type semiconductor layer needs to be removed. Therefore, when forming the transparent conductive film on the p-type semiconductor layer, the p-type semiconductor layer is usually subjected to surface treatment using a cleaning solution to remove an oxide.
However, since a transparent conductive film is formed above the insulation film in the semiconductor light-emitting device having the insulation film under the p-side electrode, the insulation film is formed on the p-type semiconductor layer earlier than the transparent conductive film. When the surface of the p-type semiconductor layer with the insulation film formed thereon is subjected to surface treatment using a cleaning solution to remove an oxide, the insulation film will be removed with the cleaning solution. Therefore, in the semiconductor light-emitting device having the insulation film under the p-side electrode, a cleaning solution less capable of removing an oxide should be used in the surface treatment of the p-type semiconductor layer before forming the transparent conductive film. As a result, the ohmic contact between the transparent conductive film and the p-type semiconductor layer is less likely to be obtained. This also applies to a semiconductor light-emitting device having a transparent conductive film above an n-type semiconductor layer.
In the semiconductor light-emitting device disclosed in Patent Literature 4, the transparent conductive film is formed on a p-type semiconductor layer, and an insulation film is formed on the transparent conductive film. In this semiconductor light-emitting device, the transparent conductive film is formed on the p-type semiconductor layer earlier than the insulation film.
However, since transparent conductive films are formed on and under the insulation film, the path of current flowing through the semiconductor light-emitting device changes depending on the relation between the layer thickness of the transparent conductive film formed on the insulation film and that of the transparent conductive film formed under the insulation film. Thus, the emission characteristics vary among semiconductor light-emitting devices unless the layer thicknesses of the transparent conductive films formed on and under the insulation film are controlled strictly.